Vacuum tube (or valve) amplifiers have used cathode-coupled input stages to generate complementary output signals from a single-ended input signal. Such complementary output signals can be used to drive efficient push-pull output stages. Symmetry in the amplitude (and in the phase) of the complementary output signals has been shown to suffer unless unequal plate load resistors are employed. Mullard (of England) and Acrosound (Keroes Enterprises, of the United States) have utilized this technique with some degree of success.
In general, triode devices have been used to form cathode-coupled input pairs. However, the use of higher-gain pentode devices is desirable as these devices can allow a preferred voltage gain, phase inversion, and voltage drive to be achieved in a single amplifier stage. Achieving symmetry of the complementary output signals using unequal plate load resistors has been problematic with pentode-based cathode-coupled input stages. Also, symmetry has not been achieved using simple screen coupling through a capacitive network. Quad (of England) utilized a technique that takes a small portion of the signal from the “upper” tube in a cathode-coupled pair, and feeds the portion to the grid of the “lower” tube of the pair. This technique can affect both the bandwidth and maximum voltage swing of the stage, and can introduce distortion, generated in the upper tube, into the lower tube of the pair.
When driving push-pull output stages with complementary output signals, it is important that the net current flow through an output transformer coupled to the push-pull stage be zero. The net current flow is the difference between the currents flowing from the center tap of the transformer primary winding to each plate connection of the push-pull stage (or plate connection and screen tap in the case of Ultralinear transformers). Offset (or inequality) in these currents can create DC-induced magnetization of the transformer core.
Ideally, a transformer core maintains a neutral magnetic flux with variations created only by an AC (e.g., audio) signal passing through it. Unequal current flows from power tube plates of the push-pull stage to the center tap of the transformer can create a static flux, which can shift the intersection point of the core's B-H curve. This can result in increased harmonic distortion in the transformer, particularly at signal frequencies that approach the low frequency limits of the transformer or at signal levels that approach the power limits of the transformer.